Bluecoats the Vol.3: the Skyriders, Paperback/Raoul Cauvin

Blutch and Chesterfield go up in an observation balloon to spy on the Confederate troops. When our heroes crash-land on their superiors, the generals send them behind enemy lines to try and free Stark, who has been captured on a previous sortie. A tricky mission, not only for our heroes, but also for the other side.